Can you play a hidden card at a location where you already have a hidden card to replace/get rid of the older hidden card?

Ruling: No, you cannot hide a card at a battlefield where you already have a hidden card to implicitly overwrite it. You must first play (reveal) the existing hidden card and let it resolve before you can hide another card at that battlefield.

Sequence:

• Play and resolve the existing hidden card first
• Then you can hide a new card at that battlefield

Nuances:

• Each battlefield has its own facedown zone which can only contain 1 hidden card per player (except Bandle Tree which allows 2)
• You can only hide a card to a battlefield you control
• Hiding a card is base speed and does not use the chain
• Hidden cards can be played anytime you could play a reaction, starting the turn after it's hidden
• Revealing a hidden card counts as playing it (triggers play effects)